1. Field of the Invention
The present invention relates to a conversion device for converting a distance-measuring pulse obtained in a laser interferometic measuring device into a distance signal representing a distance in a prescribed measuring unit system such as the metric system.
2. Description of the Prior Art
A laser measuring machine is often used in measuring the distance between two points. In particular, a laser interferometic measuring machine is suitable for accurate measuring, and this type of measuring machine usually produces a distance-measuring signal as a pulse signal for every reference distance of .lambda./N depending on the wavelength .lambda. of the laser, where N is a positive integer. On the other hand, apparatus utilizing a result of measurment by the laser measuring machine usually uses the metric system, and therefore, the pulse signal outputted from the laser measuring machine is required to be converted into a signal represented in a prescribed measuring unit system such as the metric system.
According to a first method of converting a unit system which is known in the art, the distance-measuring pulse outputted from a laser interferometic measuring machine is multiplied by a conversion coefficient representing the reference distance of .lambda./N in the metric system.
Japanese Patent Laying Open Gazette No. 55-69004 discloses a second method where the distance-measuring pulse outputted from a laser measuring machine is divided by a rate-multiplier counter with a prescribed dividing ratio to thereby convert the distance-measuring pulse into a signal of the metric system. In the second method, when the reference distance of .lambda./N is 0.0791 .mu.m and a unit length in the metric system is set to be 0.1 .mu.m, the dividing ratio is set to be 7910/10.sup.4. In this case, when ten pulses of the distance-measuring signal are supplied to the rate-multiplier counter, a signal of eight pulses is produced to indicate that a distance-to-be-measured is about 0.8 .mu.m.
According to the first method, since multiplication is performed in the unit conversion operation, a computation speed is reduced as the number of significant digits increases in order to express distance-measuring data more accurately. Accordingly, when a distance-to-be-measured continuously changes at a high speed, it is difficult to obtain distance-measuring data in response to the rapid change of the distance-to-be-measured.
According to the second method, it is required to increase the number of counters in the rate-multiplier in order to improve accuracy of approximation. This increases the cost of the device and reduces the computation speed.